Mast cells (MCs) are recognized as essential effector cells in the elicitation of allergic response by releasing cytoplasmic granule remnants. Previous studies suggest that MCs also participate in leukocyte recruitment, foam cell formation, smooth muscle cell proliferation, and T cell migration and activation, all essential for atherosclerosis, an inflammatory disease involving sophisticated cell-cell interaction, cytokine production, and immune responses. Detection of increased MC accumulation in human atherosclerotic lesions suggests their participation in atherogenesis. MCs appear in highest density in atheroma shoulder regions, where they are prone to erosion and rupture, but appear only scarcely in the fibrous caps and unaffected aortae. However, it remains uncertain whether accumulation of MCs in atheromata directly affects atherogenesis or is merely one of the many hallmarks. Our recent preliminary studies suggest that MCs participate directly in atherogenesis. In mice, the accumulation of lesion MCs correlated with the progression of atherogenesis. Absence of MCs impaired atherogenesis by >50% after 26 weeks of an atherogenic diet. Reduced atheromata in MC-null mice can be fully restored by reconstituting the mice with wild-type MCs but not interleukin-6-deficient MCs. Furthermore, MC activation with MC destabilizer enhanced atherogenesis by nearly 30%. Lack of mechanistic explanations for these observations led to a central hypothesis that MCs directly affect atherogenesis by releasing inflammatory mediators to promote leukocyte recruitment and migration, macrophage foam cell formation, smooth muscle cell proliferation and migration, lymphocyte activation, and matrix-degrading protease expression. We propose three specific aims to test this hypothesis by examining whether absence of MCs affects atherogenesis and whether we can control atherosclerosis by regulating MC activation; identifying which molecules MCs use to influence vascular wall remodeling; and testing how MCs affect other vascular cells in vitro; whether absence of MCs affects the biology of other hematopoietic cells; and which mediators are required for MC migration and recruitment in vivo. Together, these lines of experiments should provide both in vitro and in vivo evidence of whether the pathogenesis of atherosclerosis requires MC functions, and result in detailed mechanistic explanations of how MCs might participate in the pathogenesis of this common vascular disorder. [unreadable] [unreadable] [unreadable]